Strippable semiconductive composition comprising low melt temperature polyolefin

ABSTRACT

The present invention relates to a semiconductive polymer composition having easily controllable stripping characteristics, especially for an electric power cable. The semiconductive polymer composition comprises an ethylene copolymer comprising polar co-monomer units, wherein the amount of the polar co-monomer units in the ethylene copolymer is 10 wt. % or more, based on the total weight of the ethylene copolymer; an olefin copolymer comprising propylene monomer units and monomer units of an alpha-olefin having at least 4 carbon atoms, wherein the olefin copolymer has a melting point of 110° C. or less, and carbon black in an amount of from 10 to 50 wt. %, based on the total weight of the semiconductive polymer composition, wherein the olefin copolymer (B) is prepared by using a metallocene polymerisation catalyst. The composition provides improved strippability and enhanced mechanical as well as electrical property profile.

The present invention relates to a semiconductive polymer compositionhaving easily controllable stripping characteristics, especially for anelectric power cable. The present invention further relates to the useof such a semiconductive polymer composition and an electric power cablecomprising at least one layer comprising said semiconductive polymercomposition.

Electric power cables for medium to high voltages normally include oneor more metal conductors surrounded by an insulating material like apolymer material such as an ethylene polymer.

In power cables, the electric conductor is usually coated first with aninner semiconducting layer, followed by an insulating layer, then anouter semiconducting layer, followed by optional layers such aswater-barrier layers and on the outside optionally a sheath layer. Thelayers of the cable are commonly based on different types of ethylenepolymers.

The insulating layer and the semiconducting layers normally consist ofethylene homo- and/or copolymers which are preferably crosslinked. LDPE(low density polyethylene, i.e. polyethylene prepared by radicalpolymerization at a high pressure) crosslinked by adding peroxide, e.g.dicumyl peroxide, in connection with the extrusion of the cable, hasrecently become the predominant cable insulating material. The innersemiconducting layer normally comprises an ethylene copolymer, such asan ethylene-vinyl acetate copolymer (EVA), ethylene methylacrylatecopolymer (EMA), ethylene ethylacrylate copolymers (EEA), ethylenebutylacrylate copolymer (EBA). The composition of the outersemiconducting layer differs depending on whether it has to bestrippable or not.

A common concept for making an semiconductive layer strippable from anusually non-polar insulation layer is to increase the polarity of thesemiconductive layer. This is e.g. done by the addition of highly polaracrylonitrile-butadiene rubber (NBR) to the semiconductive compositionwhich e.g. further comprises an ethylene copolymer, such as anethylenevinyl acetate copolymer (EVA) and sufficient carbon black tomake the composition semiconducting.

As an example of a strippable composition, mention may be made ofEP-B1-0 420 271 which discloses a semiconducting insulation shieldingcomposition for electric cables which, based on the total weight of thecomposition, consists essentially of (A) 40-64% by weight of anethylene-vinyl acetate copolymer with 27-45% of vinyl acetate, (B) 5-30%by weight of an acrylonitrile-butadiene copolymer with 25-55% ofacrylonitrile, (C) 25-45% by weight of carbon black having a surfacearea of 30-60 m²/g, and (D) 0.2-5% by weight of an organic peroxidecrosslinking agent. In addition, the composition may include 0.05-3% byweight of conventional additives.

As a further example of prior art, strippable semiconductingcompositions for electric cables, mention may be made of U.S. Pat. No.4,286,023 which discloses a polymer composition for electric cablescomprising (A) an ethyl-ene copolymer selected from the group consistingof ethylene-alkyl acrylate copolymers containing about 15-45% by weightof alkyl acrylate, said alkyl acrylate being selected from the groupconsisting of C₁-C₈ alkyl esters of (meth)acrylic acid, such as methylacrylate, ethyl acrylate, methyl methacrylate, butyl acrylate,2-ethyl-hexyl acrylate and the like, and ethylene-vinyl acetatecopolymers containing about 15-45% by weight of vinyl acetate, (B) abutadiene-acrylonitrile copolymer (nitrile rubber) containing about10-50% by weight of acrylonitrile (C) conductive carbon black, and (D) aperoxide crosslinking agent, wherein the weight ratio A:B=1:9 to 9:1;C:(A+B)=0.1 to 1.5, and D is present in an amount of 0.2-5% by weight ofthe total composition.

Although prior art compositions for semiconducting layers in electriccables are satisfactory for many applications, there is always a desireto improve their characteristics and eliminate or reduce anydisadvantages they may have.

Furthermore, WO 99/20690 discloses an inner semiconducting compositionfor electric cables which, based on the total weight of the composition,comprises

-   -   (a) 30-90% by weight of an ethylene copolymer,    -   (b) carbon black to make the composition semiconducting,    -   (c) 0-8% by weight of a peroxide crosslinking agent,    -   (d) 0-8% by weight of conventional additives,        wherein the ethylene copolymer (a) is an        ethylene-methyl(meth)acrylate copolymer. It is reported that the        use of EMA improves the thermostability of the polar copolymer        and the composition containing the same. Thus, the composition        can be heated to higher temperatures, e.g. during compounding        and crosslinking in other known compositions. Consequently, a        higher production rate during compounding and a higher line        speed during cable production is possible.

It is the object of the present invention to provide a newsemiconductive polyolefin composition suitable for a semiconductivelayer of a power cable which is strippable and which allows for an easyadjusting of its stripping characteristics according to specific needs.

Moreover, it is a further object of the present invention to provide astrippable semiconductive copolymer composition which is easilyprocessable, which has sufficient thermo-oxidative stability,compounding consistency and has improved handling properties.

The above objects are achieved by the present invention by providing asemiconductive polymer composition comprising

-   (A) an ethylene copolymer comprising polar co-monomer units, wherein    the amount of the polar co-monomer units in the ethylene copolymer    is 10 wt. % or more, based on the total weight of the ethylene    copolymer,-   (B) an olefin copolymer comprising propylene monomer units and    monomer units of an alpha-olefin having at least 4 carbon atoms,    wherein the olefin copolymer has a melting point of 110° C. or less,    and-   (C) carbon black in an amount of from 10 to 50 wt. %, based on the    total weight of the semiconductive polymer composition,    wherein the olefin copolymer (B) is prepared by using a metallocene    polymerisation catalyst.

Furthermore, the present invention provides a power cable comprising aconductor, an insulating layer and at least one semiconductive layerwherein at least one semiconductive layer comprises the semiconductivepolymer composition as mentioned above.

In the art it is common knowledge that for such a strippablesemiconductive layer adjacent to an insulating layer which usually madeof a rather non-polar composition e.g. a polyethylene composition, acomparatively polar polymer composition is to be used, so as tofacilitate the stripping.

It has now surprisingly been found that contrary to the expectations inthe art, an olefin copolymer as defined in the present invention ishighly suitable in a semiconductive layer of a power cable to enableeasy stripping of the semiconductive layer from an adjacent insulatinglayer. Furthermore, the strippability of the semiconductive compositionof the invention can easily be adjusted according to e.g. differentindustry standards.

Still further, it is an advantage of the present invention that thesemiconductive composition does not comprise NBR, which has achewing-gum like consistency and thus gives rise to handling and dosingproblems. Usually, NBR is produced and shipped in large bundles whichmust, prior to compounding, be grained or cut into smaller pieces in aspecial processing step. Therefore, the use of NBR is inconvenient forcontinuously feeding it into the compounding mixer.

The olefin copolymer used in the present invention can easily behandled, e.g. in the form of pellets, and can easily be dosed uponformation of the composition without any special graining or cuttingprocessing step. Hence, the compounding process and the compoundingconsistency and consequently the product consistency are very muchimproved by replacing NBR.

Also the thermo-oxidative properties of the composition are alsoimproved, which leads to less degradation upon processing. NBR containsa lot of double bonds that easily reacts with oxygen in the air, duringthe compounding step resulting in thermo-oxidative degradation whichmight result in lump formation in the final composition. The olefincopolymer contains a limited number of double bonds and is not so easyto oxidize leading to less degradation upon processing and in a moreconsistent product. Alternatively, the improved thermo-oxidativeproperties can be utilized using higher processing temperature e.g.higher throughput in the compounding machine.

Finally, the olefin copolymer matches better with a base resin duringcompounding, leading to a simpler formulation which shows substantiallydecreased stickiness. In a composition containing very sticky NBR acompabilitiser, lubricants like waxes, stearates or silicones etc. andparting agent are necessary to get a homogeneous final free flowingcomposition. When using the olefin copolymer in the compositionaccording to the present invention, these disadvantages due to thestickiness of NBR are overcome, thus the above additives may be omittedor their amount may be reduced.

The invention is characterized i.a. by using a metallocenepolymerization catalyst in the preparation of the olefin copolymer (B).It was surprisingly found that the inventive selection of a metallocenepolymerization catalyst provides for superior mechanical properties aswell as processing properties of the final semiconductive polymercomposition.

With the use of a metallocene polymerization catalyst it hassurprisingly become possible to improve the property profile of astrippable semiconductive polymer composition compared to asemiconductive polymer composition prepared by a conventional Zieglercatalyst. In particular, the temperature cable manufacture processingwindow is much higher when using a metallocene catalyst. Thus, theprocessing temperature may be sufficiently low to avoid scorchgeneration due to peroxide decomposition and sufficiently high toprovide for superior melt homogenisation so as to guarantee an elasticformable melt which gives an excellent surface smoothness to the polymercomposition. Superior melt homogenisation may be preferably obtained bya melt temperature in an extruder of more than 25° C. above the meltingpoint of the polymer component comprising the highest melting point.

A metallocene catalyst further lowers the Vicat softening point of theobtained olefin copolymer (B), ensures an even comonomer distributionand a narrow molecular weight distribution (Mw/Mn) which parameterscontribute to the above-described enhanced property profile.

The inventive semiconductive polymer composition also needs lessprocessing aid such as wax, parting agent (anti-caking agent), comparedto compositions comprising NBR. Due to the decreased stickiness of theinventive polymer compositions compared to conventional strippablecompositions using NBR as a component, the new compositions of thepresent invention can be stripped at higher temperatures up to at least75° C. without any problems in strippability at comparatively low stripforces. Furthermore polymer pellets may be produced which do not sticktogether even at temperatures up to 70° C. The above processing aids andparting agents which are indispensable in compositions containing NBRmay thus be omitted.

Preferably, the olefin copolymer (B) in the semiconductive compositioncomprises 40 wt. % or less, still more preferably comprises 30 wt. % orless of monomer units of an alpha-olefin having at least 4 carbon atoms.It is further preferable that the olefin copolymer (B) comprises 5 wt. %or more, monomer units of an alpha-olefin having at least 4 carbonatoms.

The alpha-olefin for incorporating one or more monomeric units into theolefin copolymer (B) may preferably be selected from the groupconsisting of butene, pentene, hexene, heptene, octene, nonene anddecadiene.

The olefin copolymer (B) may also incorporate, in addition to thealphaolefin, a monomeric unit of ethylene with a preferred content ofless than 10%.

The propylene unit may preferably be contained in the olefin copolymer(B) in an amount of 50 wt. % or more, more preferably in an amount offrom 70 to 90 wt. %, based on the total weight of the olefin copolymer(B).

In a preferred embodiment the olefin copolymer consists of propylene andbutene monomer units.

The melting point of the olefin copolymer preferably is 110° C. or less.

Furthermore, the melting point of the olefin copolymer (B) preferably is100° C. or less, more preferably 90° C. or less, even more preferably85° C. or 75° C. or less. The melting point of the olefin copolymer (B)should preferably not be lower than a range of from 50 to 55° C.

Preferably, the semiconductive composition comprises the olefincopolymer (B) in an amount of 3 wt. % or more, more preferably of 5 wt.% or more, and most preferably of 10 wt. % or more.

Furthermore, the semiconductive composition preferably comprises theolefin copolymer (B) in an amount of 45 wt. % or less, more preferablyof 35 wt. % or less, and most preferably of 25 wt. % or less. Accordingto preferred embodiments of the invention the olefin copolymer (B) maybe contained in an amount of from 10 to 45 wt. %, or from 5 to 15 wt. %of the total weight of the polymer composition.

The melt flow rate MFR₂, measured at 230° C., of the olefin copolymer(B) preferably is from 0.5 to 50 g/10 min, more preferably is from 3 to35 g/10 min.

In one embodiment of the semiconductive composition of the invention,the olefin copolymer comprises or consists of a propylene copolymer,preferably a random propylene copolymer. The propylene copolymer maycomprise a random propylene copolymer matrix with a dispersed rubberphase, such as an propylene-alpha-olefin rubber. One example is apropylene-butene rubber. The propylene copolymer may also comprise anolefin-based terpolymer such as a propylene-ethylene-alpha-olefinterpolymer. One example is a propylene-ethylene-butene terpolymer whichmay have elastomeric properties.

The olefin copolymer (B) may preferably have a density of 910 kg/cm³ orlower, more preferably 900 kg/cm³ or lower according to ASTM D792.

The semiconductive copolymer composition of the present inventionpreferably further comprise a polar copolymer (A). Such a polarcopolymer may be any of the conventionally used polar copolymers insemiconductor copolymer compositions for power cables.

Preferably, the polar copolymer (A) is a polar olefin copolymer, morepreferably a polar ethylene copolymer.

The polar comonomers contained in the polar copolymer (A) may beselected from acrylic acids, methacrylic acids, acrylates,methacrylates, acetates and/or vinyl esters.

Preferably, the polar copolymer (A) is a copolymer of ethylene withunsaturated carboxylic acid esters having preferably 4 to 8 carbon atomsor vinyl esters, preferably C₁₋₄ acrylates, such as methyl, ethyl,propyl or butyl (meth-) acrylates, or vinyl acetate.

Furthermore, the polar copolymer (A) may be present in thesemiconductive composition in an amount of 65 wt. % or less, morepreferably of 60 wt. % or less, still more preferably of 55 wt. % orless, and most preferably of 50 wt. % or less, based on the totalcomposition.

The polar copolymer (A) preferably has a amount of polar comonomer unitsof 10 wt. % or more, more preferably of 20 wt. % or more, and mostpreferably of 25 wt. % or more.

Especially, an ethylene/vinyl acetate copolymer may be used. Thecopolymer may be composed of ethylene and a vinyl ester having 4 or 5carbon atoms as main constituents. The vinyl ester preferably may bevinyl acetate, vinyl propionate or a mixture of these.

Preferably, the polar copolymer (A) has a melt flow rate MFR₂ (2.16kg/190° C.) of 0.1 to 100 g/10 min, more preferably 1 to 60 g/10 min,even more preferably 5 to 50 g/10 min, and most preferably 15 to 50 g/10min.

It is further advantageous, if the amount of the olefin copolymer (B)with respect to the sum of the polar copolymer (A) and the olefincopolymer (B) is preferably not more than 45 wt. %, more preferably notmore than 25 wt. %, even more preferably not more than 20 wt. %. If theamount of the olefin copolymer (B) is greater than the above ranges, thestickiness and processability of the total semiconductive polymercomposition may be deteriorated.

The semiconductive polymer composition further preferably comprisescarbon black.

The amount of carbon black is at least such that a semiconductingcomposition is obtained. Depending on type of the used carbon black andthe desired use and conductivity of the composition, the amount ofcarbon black can vary.

Preferably, the polymer composition comprises 10 to 50 wt % carbonblack, based on the weight of the total semiconductive composition. Morepreferably, the amount of carbon black is 10 to 45 wt. %, still morepreferably 15 to 45 wt. % or 20 to 45 wt. %, still more preferably 30 to45 wt. %, still more preferably 35 to 45 wt. %, and most preferably 36to 41 wt. %.

Any carbon black can be used which is electrically conductive. Examplesof suitable carbon blacks include furnace blacks and acetylene blacks.As carbon black, furnace carbon black is especially preferred.

Suitable furnace blacks may have a primary particle size of greater than28 nm, measured according to ASTM D-3849. Many suitable furnace blacksof this category are characterized by an iodine number between 30 and200 mg/g according to ASTM D-1510 and an oil absorption number between80 and 300 ml/100 g according to ASTM D-2414.

Other suitable carbon blacks can be made by any other process or befurther treated.

Suitable carbon blacks for semiconductive cable layers are preferablycharacterized by their cleanliness. Therefore, preferred carbon blackshave an ash-content of less than 0.2 wt. % measured according toASTM-1506, a 325 mesh sieve residue of less than 30 ppm according toASTM D-1514 and have less than 1 wt. % total sulphur according toASTM-1619.

Most preferred are extra-clean furnace carbon blacks having anash-content of less than 0.05 wt. % measured according to ASTM-1506, a325 mesh sieve residue of less than 15 ppm according to ASTM D-1514 andhave less than 0.05 wt. % total sulphur according to ASTM-1619.

According to a preferred embodiment, the semiconductive polymercomposition further comprises a crosslinking agent.

In the context of the present invention, a crosslinking agent is definedto be any compound which can initiate radical polymerisation. Acrosslinking agent can be a compound capable of generating radicals whendecomposed but also comprises the radicals obtained after decomposition.Preferably, the crosslinking agent contains at least one —O—O— bond orat least one —N═N— bond. More preferably, the cross-linking agent is aperoxide and/or a radical obtained therefrom after thermaldecomposition.

The cross-linking agent, e.g. a peroxide, is preferably added in anamount of less than 3.0 wt. %, more preferably 0.2 to 2.6 wt. %, evenmore preferably 0.3 to 2.2 wt. %, based on the weight of the polymercomposition. To have a good balance between scorch and crosslinkingefficiency, it might be preferred to add the crosslinking agent, inparticular a peroxide, in an amount of 0.4 to 1.5 wt. %, even morepreferably 0.8 to 1.2 wt. %, based on the weight of the semiconductivecomposition.

The cross-linking agent may be added to the semiconductive compositionduring the compounding step (i.e. when the unsaturated polyolefin ismixed with the carbon black), or after the compounding step in aseparate process, or during the semiconductive crosslinkable compositionis extruded, or after the extrusion, e.g. by diffusion of cross-linkingradicals from another cable layer into the semiconductive layer.

As peroxides used for crosslinking, the following compounds can bementioned: di-tert-amylperoxide,2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,2,5-di(tert-butylperoxy)-2,5-dimethylhexane, tert-butylcumylperoxide,di(tert-butyl)peroxide, dicumylperoxide,di(tert-butylperoxyisopropyl)benzene,butyl-4,4-bis(tert-butylperoxy)valerate,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,tert-butylperoxybenzoate, dibenzoylperoxide.

Preferably, the peroxide is selected from2,5-di(tert-butylperoxy)-2,5-dimethyl-hexane,di(tert-butylperoxy-isopropyl)benzene, dicumylperoxide,tert-butylcumylperoxide, di(tert-butyl)peroxide, or mixtures thereof.Most preferably, the peroxide is tert-butylcumylperoxide

The semiconductive polymer composition may comprise further additives.As possible additives, antioxidants, scorch retarders, crosslinkingboosters, stabilisers, processing aids, lubricants, flame retardantadditives, acid scavengers, inorganic fillers, voltage stabilizers,additives for improving water tree resistance, or mixtures thereof canbe mentioned.

It is also possible to include a small amount, preferable 15% or less,of another filler in addition to the carbon black to improve propertieslike tear behaviour etc. The filler can also act as an acid scavenger.Ethylene vinylacetate used as a copolymer in the semiconductiveformulation starts to degrade above 150° C. resulting in formation ofacetic acid which provokes an increased risk for corrosion of processingequipment. Preferably inorganic fillers will neutralise the acid andreduce the acid corrosion attack. Suitable filler materials may beselected from the group consisting of calcium carbonate, talc, mica,wollastonite, barium sulfate, calcite, and hydrotalcite.

In the production of a power cable comprising a conductor, an insulatinglayer and at least one semiconductive layer, the inventivesemiconductive copolymer composition may be contained in at least one ofsaid semiconductive layers.

The power cable comprising the semiconductive copolymer composition ofthe present invention may further comprise additional layers such aswater barrier layers and a sheath layer.

As mentioned above, a crosslinking agent, preferably a peroxide, can beadded to the semiconductive polyolefin composition. The point in timefor adding the crosslinking agent can be varied. As an example, thecrosslinking agent may be added to the semiconductive crosslinkablepolymer composition when the polyolefin is mixed with the carbon blackin a compounding step, or after the compounding step in a separateprocess step. Furthermore, the crosslinking agent may be added duringextrusion of the semiconductive crosslinkable polymer composition.

As a further alternative, the crosslinking agent can be added duringand/or after application of the semiconductive crosslinkable polymercomposition onto the substrate. In this preferred embodiment, thecrosslinking agent can be provided in an external reservoir from whichit can migrate into the layer comprising the semiconductivecrosslinkable composition. In the context of the present invention, an“external reservoir” is a reservoir which is not part of the layercomprising the semiconductive crosslinkable composition. Preferably, theexternal reservoir is another layer also applied onto the substrate andcontaining the crosslinking agent. As explained above, the term“crosslinking agent” has to be defined in a broad sense. Thus, the otherlayer acting as a reservoir may comprise compounds not yet decomposedbut may also comprise radicals resulting from decomposition. From theother layer, the crosslinking agent migrates to the layer comprising thesemiconductive crosslinkable composition. Thus, since the crosslinkingagent is provided from an external reservoir during and/or after havingbeen applied onto the substrate, the semiconductive crosslinkablepolymer composition of the present invention can be extruded withoutcrosslinking agent or at least with a very low amount of crosslinkingagent.

In a preferred embodiment, the other layer acting as an externalcrosslinking agent reservoir is provided adjacent to the layercomprising the semiconductive crosslinkable polymer composition tofacilitate migration of the crosslinking agent. If necessary, migrationis enhanced by thermal treatment of one of these layers or both layers.

When sufficient crosslinking agent has been diffused into thesemiconductive crosslinkable composition, said composition can betreated under crosslinking conditions. If peroxides are used,crosslinking can be effected by raising the temperature to at least 160to 170° C.

Usually, the semiconductive composition is processed at a temperature ofat most 140° C., more preferably at a temperature of at most 135° C.

Processing comprises both a compounding step of the composition as wellas the extrusion into a layer of a cable.

The present invention also pertains to an electric power cablecomprising a semiconducting layer formed by the semiconductivecomposition as described above.

Usually, semiconducting layers are contained in medium to high voltagecables, in which a conductor core, e.g. copper or aluminum, issurrounded by an inner semiconducting layer, an insulation layer, and anouter semiconducting layer. Optionally, further shielding layers and/ora cable jacket may be present.

Preferably, at least the outermost semiconductive layer of a power cableis formed by the composition as described above.

Furthermore, preferably the insulation layer comprises an ethylene homo-or copolymer, which is preferably crosslinked.

Insulations can consist of extruded polymers included polyethylene (LDPEand HDPE), cross-linked polyethylene (XLPE), which may be water-treeresistant (WTR-XLPE) and ethylene propylene rubber (EPR). The extrudedpolymers may either be thermoplastic or thermoset. Thermoplasticmaterial will deform upon subsequent heating, wheras thermoset materialwill tend to maintain their form at higher operating temperatures.

Finally, the present invention relates to the use of a semiconductingpolymer composition as described above for the production of asemiconductive layer of an electric power cable, preferably a medium tohigh voltage electric power cable.

EXAMPLES

The present invention will now be described in more detail by referenceto the following examples and comparative examples. Parts and % areweight based, if not specified otherwise.

1. Test Methods

Unless otherwise stated in the description or claims, the followingmethods were used to measure the properties defined generally above andin the claims and in the examples below. The samples were preparedaccording to given standards, unless otherwise stated.

(a) Melt Flow Rate

The melt flow rate was determined for propylene homo- and copolymersaccording to ISO 1133 at 230° C., at a 2.16 kg load (MFR₂).

(b) Density

The density of the materials and compositions was determined accordingto ASTM D792 and is given in kg/m³.

(c) Melting Temperature

The melting temperature (T_(m)) of the olefin copolymer was determinedaccording to ASTM D 3418. T_(m) was measured with a Mettler TA 820differential scanning calorimetry (DSC) apparatus on 3±0.5 mg samples.Melting curves were obtained during 10° C./min cooling and heating scansbetween −10 to 200° C. Melting temperatures were taken as the peak ofendotherms and exotherms.

(d) Comonomer Content of the Polar and Alpha-Olefin Copolymer

Comonomer content (wt. %) of the polar comonomer was determined in aknown manner based on Fourier transform infrared spectroscopy (FTIR)determination calibrated with ¹³C-NMR as described in Haslam J, Willis HA, Squirrel D C. Identification and analysis of plastics, 2^(nd) ed.London lliffe books; 1972. FTIR instrument was a Perkin Elmer 2000,lscann, with a resolution of 4 cm⁻¹. The peak obtained for the testedcomonomer was compared to the peak of polyethylene as evident for askilled person (e.g. the peak for butyl acrylate at 3450 cm⁻¹ wascompared to the peak of polyethylene at 2020 cm⁻¹). The wt. % wasconverted to mol % by calculation, based on the total moles ofpolymerisable monomers.

An alternative method to determine polar as well as the alpha-olefincomonomer content is to use an NMR-method which would give equal resultsto above X-ray and FTIR method. The following method, for example, givesresults which are, for the purposes of the invention, equivalent tothose above:

The comonomer content was determined by using ¹³C-NMR. The ¹³C-NMRspectra were recorded on Bruker 400 MHz spectrometer at 130° C. fromsamples dissolved in 1,2,4-trichlorobenzene/benzene-d6 (90/10 w/w).

(e) Hot Set Elongation

For verification of proper curing of the different layers in the cableconstructions, the hot set elongation and permanent set were determinedaccording to IEC 60811-2-1, by measuring thermal deformation at 200° C.and at a load of 0.2 MPa using a cable layer sample consisting of saidcrosslinked polyolefin composition of the invention

Two dumb-bell test samples were prepared from a crosslinked cable layerconsisting of a polyolefin composition to be tested by cutting aapproximate 1.0 mm thick layer sample from the test cable layer in thedirection along the cable axis. The other dimensions were according tosaid standard. In the examples given below, the test layer sample wastaken from the outer semiconductive layer of the test cable by peelingsaid outer layer having a layer thickness of 1.0 mm from the insulationlayer.

Each test sample was fixed vertically from upper end thereof in the ovenand the load of 0.2 MPa was attached to the lower end of each test layersample. After 15 min. at 200° C. in the oven, the distance between thepremarked lines was measured and the percentage hot set elongation wascalculated, resulting in elongation %. For permanent set %, the tensileforce (weight) was removed from the test samples, then recovered at 200°C. for 5 minutes and allowed to cool at room temperature until ambienttemperature was reached. The permanent set % was calculated from thedistance between the marked lines.

(f) Strip Force

Cable samples of 30 cm of length were cut in cross sectional directionfrom a test cable which had an inner semiconductive layer with athickness of 0.8±0.05 mm, an insulation layer with a thickness of5.5±0.1 mm, and an outer semiconductive layer with a thickness of 1±0.1mm. The test cables were prepared according to the method “Test cableand preparation method thereof” described below using the given innersemiconductive layer material and insulation layer material for the testsample and using the polyolefin composition to be tested as said outersemiconductive layer material. The strip force test can be made for testcable wherein said sample is in non-crosslinked or crosslinked form. Thecable samples were conditioned not less than 16 hours at 23° C. and 55%relative humidity. Two cuts of 10 cm length and 10 mm apart from eachother were applied with a knife through the outer semiconductive layerof said test cable in axial direction in such a depth to obtain a cutthickness corresponding to the thickness of said outer semiconductivelayer (1 mm). The separation of the cut of the outer semiconductivelayer was initiated manually at the cut end of the cable sample. Thecable was fixed to Alwetron TCT 25 tensile testing instrument(commercially available from Alwetron). The manually separated cut partwas clamped onto a wheel assembly which is fixed to a moveable jaw ofsaid instrument. The rotation of the wheel assembly causes theseparation of the jaws, and thus the peeling, i.e. separation, of saidsemiconductive layer from said insulation layer to occur. The peelingwas carried out using a peeling angle of 90° and peeling speed of 500mm/min. The force required to peel said outer semiconductive layer fromthe insulation was recorded and the test was repeated at least ten timesfor each test layer sample. The average force divided by the width (10mm) of the sample was taken as said strip force and the given values(kN/m at 90° C.) represent the average strip force of the test samples,obtained from at least ten tests.

(g) Spiral Strip Test

A spiral strip test or removable field test was performed on crosslinked20 kV cables under simulation of conditions for stripping the cables infield in warm climate. The conditions were changed by varyingtemperatures at which the test was performed. The test was conducted inthe following manner:

Two parallel helical scores around the cable were made in the insulationshield to a depth of approximate 0.025 mm less than the specimen(insulation shield) minimum point thickness. The distance between theparallel scores were 10 mm. The cable samples were tempered in a Hereuslab oven at specified temperature. After not less than 1 h the cablesamples were taken out of from the oven and the insulation shieldspecimen were immediately removed from the insulation by hand at a timewhen the cable still tempered. The insulation shield was subjected to a“pass test”. The criteria for passing the test were removal of theinsulation shield without tearing, breaking or leaving residualconductive material on the insulation surface which is not easilyremovable by light rubbing. Sanding should not be required to remove theresidual material.

(h) Volume Resistivity

The volume resistivity of the semiconductive material is measured oncrosslinked polyethylene cables according to ISO 3915 (1981). Cablespecimens having a length of 13.5 cm were conditioned at 1 atm and 60±2°C. for 5±0.5 hours before measurement. The resistance of the outersemiconductive layer was measured using a four terminal system usingmetal wires pressed against the semiconductive layer. The resistance ofthe inner semiconductive layer was measured by cutting the cable intotwo halves and by removing the metallic conductor. The resistancebetween the conductive silver paste applied onto the specimen ends wasthen used to determine the volume resistivity of the innersemiconductive layer. The measurements were carried out at roomtemperature (20° C.) and at 90° C.

(i) Pellet Free Flowing Test

The stickiness among polymer pellets was tested according to a “pelletfree flowing test” in the following manner. The test intends to simulatethe press conditions expected on the bottom layer of material in a 600kg box used for supplying this kind of material. The conditions changedby varying the temperature at which the test was performed. The test wasconducted in the following manner:

500 g of pellets of the semiconductive material was weighed into a 1000ml Mason Jar. An aluminum disc was placed on top of the material and a 2kg weight was placed on top of the disc. This simulates the pressure, ofabout 1 psi (6.9·10³ Pa), exerted on the bottom layer of material in abox. The specimens were tempered in a Nereus lab oven at specifiedtemperatures. After 6 hours the jars were removed from the oven, theweight and the aluminum disc were removed and the jars were inverted for1 minute. If material remained in the jars, it was separated from thejar, weighed, and reported.

(k) Walfer Boiling Test

The walfer boiling test was performed in accordance with AEIC CS5-94.Any outer covering and the conductor were removed. A representativecross section containing the extruded conductor shield and insulationshield, was cut from the cable. The resulting walfer having a thicknessof at least 25 mils (0.64 mm) was immersed in boiling stabilizeddecahydronaphthalene with for five hours using the equipment specifiedin ASTM D2765. The walfer was then removed from the solvent and examinedfor shield/insulation interface continuity with a minimum of a 15-foldmagnification. To pass the test after the above treatment, theinsulation shield should not show any cracks and continuously adhere at360° to the insulation.

(l) Tensile Strength at Break and Elongation at Break

Both properties were measured according to ISO 527 and are given in MPaand %, respectively.

(m) Thermo-Oxidative Ageing

The change in mechanical properties before and after thermo-oxidativeageing was measured according to IEC 60811-1-2,

(n) Oil Adsorption Number, (Dibutyl Phthalate)

DBP adsorption number of the carbon black samples was measured inaccordance with ASTM D2414-06a.

(o) Iodine Number

The iodine number of the carbon black samples was measured in accordancewith ASTM D1510-07.

2. Materials

The ingredients given in the following Table 1 were used for thepreparation of the polymer compositions. All amounts are given in partsby weight.

(a) Composition of the Outer Semiconductive Layer

The components of the outer semiconductive layer composition were thoseof the polyolefin composition under test. The test polyolefincompositions used in the present experimental part were polyolefincompositions of inventive examples 1-12 and the polymer compositions ofcomparative examples 1-3 as listed in the tables below.

The preparation of the outer semiconductive layer composition waseffected by compounding the components in a Buss mixer. Accordingly, thecompounding operations were made in a 46 mm continuous Buss mixer. Thecopolymer (A) and polyolefin (B), and additives, if any, were charged tothe first hopper of the mixer. A filler-like carbon black was chargedinto the subsequent second hopper together with the additive(s) and themixing was continued at 190° C. followed by pelletising. The peroxidecomponent was charged to the pellets in a separate processing step.

(b) Production of Test Cables

The test cables were prepared using a so-called “1 plus 2 extruderset-up”, in a Mailerfer extruder, supplied by Mailerfer. Thus, the innersemiconductive layer was extruded on the conductor first in a separateextruder head, and then the insulation and outer semiconductive layerare jointly extruded together on the inner semiconductive in a doubleextruder head. The inner and outer semiconductive extruder screw had adiameter of 45 mm and the insulation screw had a diameter of 60 mm.

Each test cable was produced at a production rate of 1.6 m/min using thesame conventional production conditions e.g. crosslinking of the testcables in nitrogen in a CV-vulcanization tub. Each test cable had thefollowing properties:

Test cable construction Conductor diameter 50 mm² Al Innersemiconductive 0.8 ± 0.05 mm layer, thickness Insulation layer,thickness 5.5 ± 0.1 mm Outer semiconductive 1 ± 0.1 mm layer, thickness

TABLE 1 Raw materials used in the examples MFR 2.16 kg 190° C. DensityMelting point ASTM D1238 ASTM D792 ASTM D3418 Trade name Polymer (g/10min) (kg/cm³) (° C.) Escorene ® LD783. NP EVA 1 EVA, VA = 31% 43 955 58Escorene ® UL 02133EN2 EVA 2 EVA, VA = 33% 21 956 59 Escorene ®UL00728FF EVA 3 EVA, VA = 28%  7 952 70 Krynac ® 34.35; NBRNitrile-butadiene rubber, — 980 Mooney viscosity (ML (1 + 4) (33 wt %nitrile) 100° C. = 30 +/− 3 Tafmer ® XM 5070 Copolymer 1 PP-PB copolymer7 (230° C.) 75 Tafmer ® TX1284 Copolymer 2 PP-PB-PE copolymer 6 (230°C.) 75 Carbon black Oil adsorp nr. Iodine nr. ASTMD2414 ASTM D1510 Tradename Type (ml/100 g) (mg/g) Conductex ® 7051 CB Furnace black 115-12738-48 Additives TMQ Stabiliser TMQ (CAS 26780-96-1) Antilux ® 654Processing aid Geo Liqua-cup D-16 organic Tert-butyl cumyl peroxideperoxide Peroxide (TBCP) MFR 2.16 kg 190° C. Density Melting point ASTMD1238 ASTM D 792 ASTM D3418 (g/10 min) (kg/cm³) (° C.) LE4201 InsulationLDPE peroxide 1  923 111  LE0592 Inner EBA 20 (MFR 21.16 kg 1150 93semiconductive (g/10 min) Escorene ® is a registered trademark ofExxonMobil Corporation Krynac ® is a registered tradmark of Lanxess AGTafmer ® is a registered trademark of Mitsui Co., Ltd. Conductex ® is aregistered trademark of Columbian Chemicals Company Antilux ® is aregistered trademark of RheinChemie AG LE4201 and LE0592 are commercialavailable insulation and semiconductive products produced by Borealis

The following Table 2 lists compositions according to the invention(Example 1 to 9) and Comparative Examples (CE1 and CE2) together withselected mechanical and electrical properties.

TABLE 2 Test method Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex.9 CE 1 CE 2 Strippable evaluation EVA 1 53 50 47 44 41 53.3 50.3 47.3 56EVA 2 48.1 EVA 3 47 Copolymer 1 5 10 15 20 25 15 Copolymer 2 5 10 15 CB41 39 37 35 33 37 41 39 37 38.1 43 NBR 8.9 stabilizer 1 1 1 1 1 1 0.70.7 0.7 1 1 Processing aid 3.9 SUM 100 100 100 100 100 100 100 100 100100 100 MFR 190° C., 21.6 kg, ISO 1133 75 101 107 75 35 (g/10 min)Peroxide 0.8 0.8 0.8 0.8 0.8 0.8 1 1 1 0.8 0.8 Strip Forces 90 degree3.3 2.1 2.1 2.3 2.8 5.1 3.8 3.4 33 3.1 4.6 (kN/m) Volume resistivity ISO3915 (1981) 105 54 165 370 171 153 53 53 37 72 76 (Ohm*cm) at 20° C.Volume resistivity ISO 3915 (1981) 870 870 1681 6670 9576 1260 863 712321 260 758 (Ohm*cm) at 90° C. Mechanical properties Tensile strength atbreak ISO 527 16.5 15.9 16.2 15.4 15.9 19.3 14.5 15.2 15.5 12.8 18 (MPa)Elongation at break ISO 527 293 284 268 254 269 254 245 254 241 275 250(%) Walfer boiling test AEIC CS5-94 Pass Pass Pass Pass Pass Pass PassPass Pass Pass Pass

The above Table 2 shows that the semiconductive polymer compositionsaccording to the present invention have a well-balanced property profileincluding mechanical, stripping and electrical properties meeting therequirements for crosslinked semiconductive compositions for cables.

The following Table 3 lists compositions according to the invention andcomparative Examples together with mechanical and thermo-oxidativeageing properties.

TABLE 3 Test method Ex 2 Ex 8 CE 1 CE 2 Strippable evaluation EVA 1 5050.3 56 EVA 2 48.1 Copolymer 1 10 Copolymer 2 10 CB 39 39 38.1 43 NBR8.9 Stabilizer 1 0.7 1 1 Processing aid 3.9 SUM 100 100 100 100 Peroxide0.8 1 0.8 0.8 Mechanical properties Tensile strength at break ISO 52715.9 15.2 12.8 18 (Mpa) Elongation at break (%) ISO 527 284 254 275 250After ageing in 7 days at IEC 60811-1-2 135° C. in oven Tensile strengthat break ISO 527 16.1 14.8 13 17.8 (Mpa) Elongation at break (%) ISO 527257 249 175 219 Change in mechanical IEC 60811-1-2 properties Before andafter themo-oxidative ageing Tensile strength at break 1 3 2 1 (%)Elongation at break (%) 10 2 37 12.4 Hot set elongation (%) IEC60811-2-1 43 38 26 71 Permanent set (%) IEC 60811-2-1 5.4 0 3.5 26

The combined properties of tensile strength at break, elongation atbreak, mechanical properties after thermo-oxidative ageing were clearlyimproved according to the present invention in comparison toconventional crosslinked strippable semiconductive compositions.

According to the above-described spiral strip test, cable samples weremanufactured and tested. The results are shown in the Table 4 below.

TABLE 4 Strippable Formulations Spiral strip test Product Ex. 10 Ex. 11Ex. 12 CE 3 CB 41 39 37 38.1 EVA 1 53.3 50.3 47.3 EVA 2 48.1 Stabilizer0.7 0.7 0.7 1 Copolymer 2 5 10 15 NBR 8.9 Processing aid 3.9 Sum 100 100100 100 Peroxide 0.80 0.80 0.80 0.80 Strip Force 90° (kN/m) 3.8 3.4 3.33.1 standard deviation 0.163 0.098 0.104 0.235 Nr of tests 10 11 9 10Strip Force Spiral Cut −10° C. OK OK OK OK   20° C. OK OK OK OK   45° C.OK OK OK OK   55° C. OK OK OK OK   65° C. OK OK OK break   75° C. OK OKOK —

It can be seen from the above examples that the olefin copolymer (B)according to the present invention advantageously lowers the strip forceof semiconductive power cables comprising a strippable semiconductivepolymer composition according to the present invention even attemperatures as high as 65° C. or 75° C., whereas the compositionaccording to CE3 comprising an NBR component instead of the inventiveolefin copolymer (B) broke on stripping already at 65° C.

The following Examples 13 to 15 and Comparative Examples CE1 and CE2show the free flowing properties (measured according to theabove-described pellet free flowing test) of pellets containinginventive polymer compositions in comparison to compositions not fallingunder the invention.

TABLE 5 Pellet free flowing at different temperatures Ex. 13 Ex. 14 Ex.15 CE 1 CE 2 Strippable evaluation EVA 1 47 44 41 56 EVA 2 48.1Copolymer 1 15 20 25 CB 37 35 33 38.1 43 NBR 8.9 Stabiliser 1 1 1 1 1Processing aid 3.9 Sum 100 100 100 100 100 MFR 190° C., 21.6 kg, 70.5101 107 75 35 (g/10 min) Peroxide 0.8 0.8 0.8 0.8 0.8 Parting agent(Agent added on No No No Yes No the pellet surface to reduce stickiness)Evaluation on pellets (strippable semiconductive polymer compositions)Pellet Free flowing test at 98 99.5 100 97 98 50° C. (%) Pellet Freeflowing test at 86 95 93 65 84 60° C. (%) Pellet Free flowing test at 217 22 0 1 70° C. (%)

It can be seen that the inventive polymer composition imparts improvedfree flowing properties to the produced pellets at 50° C., 60° C. and70° C. which translates into a lower tendency of pellets to sticktogether at higher temperatures, enabling to omit any parting agent inthe composition. The compositions according to the invention thus havealso advantages in packaging and transporting the polymer pellets.

1. A semiconductive polymer composition comprising: (A) an ethylenecopolymer comprising polar co-monomer units, wherein the amount of thepolar co-monomer units in the ethylene copolymer is 10 wt. % or more,based on the total weight of the ethylene copolymer, (B) an olefincopolymer comprising propylene monomer units and monomer units of analpha-olefin having at least 4 carbon atoms, wherein the olefincopolymer has a melting point of 110° C. or less, and (C) carbon blackin an amount of from 10 to 50 wt. %, based on the total weight of thesemiconductive polymer composition, wherein the olefin copolymer (B) isprepared by using a metallocene polymerisation catalyst.
 2. Thesemiconductive polymer composition according to claim 1, wherein theolefin copolymer (B) has a content of said alpha-olefin having at least4 carbon atoms of 40 weight % or less, based on the total weight of theolefin copolymer.
 3. The semiconductive polymer composition according toclaim 1, wherein the olefin copolymer (B) has a content of saidalpha-olefin having at least 4 carbon atoms of 5 weight % or more. 4.The semiconductive polymer composition according to claim 1, wherein theolefin copolymer (B) has a melting point of 95° C. or less.
 5. Thesemiconductive polymer composition according to claim 1, wherein theolefin copolymer (B) further comprises ethylene monomer units.
 6. Thesemiconductive polymer composition according to claim 5, wherein theolefin copolymer (B) has an ethylene content of not more than 10 wt. %,of the total weight of the olefin copolymer (B).
 7. The semiconductivepolymer composition according to claim 1, wherein the olefin copolymer(B) is present in an amount of from 3 to 40 wt. % of the total weight ofthe polymer composition.
 8. The semiconductive polymer compositionaccording to claim 7, wherein the olefin copolymer (B) is present in anamount of not more than 12 wt. % of the total weight of the polymercomposition.
 9. The semiconductive polymer composition according toclaim 1, wherein the polar comonomers contained in the ethylenecopolymer (A) are selected from the group consisting of acrylic acids,methacrylic acids, acrylates, methacrylates, acetates and vinyl esters.10. The semiconductive polymer composition according to claim 1, whereinthe ethylene copolymer (A) is present in the composition in an amount offrom 30 to 65 wt. % of the total weight of the semiconductivecomposition.
 11. The semiconductive polymer composition according toclaim 1, wherein the weight ratio of the olefin copolymer (B) withrespect to the sum of the weight of the polar copolymer (A) and theolefin copolymer (B) is not more than
 20. 12. The semiconductive polymercomposition according to claim 1, further comprising a basic, inorganicfiller.
 13. A power cable comprising a conductor, an insulation layerand at least one semiconductive layer, wherein the at least onesemiconductive layer comprises a semiconductive polymer composition asdefined in claim
 1. 14. The power cable according to claim 13, whereinthe insulating layer comprises an ethylene polymer.
 15. Use of asemiconductive polymer composition according to claim 1 in thepreparation of a semiconductive layer of a power cable.